Constraining the LIGO/Virgo AGN channel with black hole spins

TL;DR

This paper investigates how gas interactions in active galactic nucleus disks can spin down merging black holes, explaining observed low spins and providing new tests for black hole dynamics in such environments.

Contribution

It introduces the idea that gas-induced spin down in AGN disks can reconcile hierarchical merger models with observed low black hole spins.

Findings

Gas interactions can reduce black hole spins after mergers.

Low observed spins do not exclude hierarchical merger scenarios.

Spin and tilt measurements test black hole dynamics in AGN environments.

Abstract

Merging black holes (BH) are expected to produce remnants with large dimensionless spin parameters ($a_{\rm spin} \sim 0.7$). However, gravitational wave (GW) observations with LIGO/Virgo suggest that merging BH are consistent with modestly positive but not high spin ($a_{\rm spin} \sim 0.2$), causing tension with models suggesting that high mass mergers are produced by hierarchical merger channels. Some BH also show evidence for strong in-plane spin components. Here we point out that \emph{spin down} of BH due to eccentric prograde post-merger orbits within the gas of an active galactic nucleus (AGN) disk can yield BH with masses in the upper mass gap, but only modestly positive $a_{\rm spin}$, and thus observations of BH with low spin \emph{do not} rule out hierarchical models. We also point out that the fraction of BBH mergers with significant in-plane spin components is a strong test of interactions between disk binary black holes (BBH) and nuclear spheroid orbiters. Spin magnitude and spin tilt constraints from LIGO/Virgo observations of BBH are an excellent test of dynamics of black holes in AGN disks, disk properties and the nuclear clusters interacting with AGN.